1. Field of the Invention
The invention relates to a method and apparatus for effecting the cooling of a stream of gas, such as air, by passing through a fluidized bed of dry ice particles. The invention has particular utility in effecting the cryogenic treatment of food articles.
2. Summary of the Prior Art
A large variety of prior art systems have been developed to effect the cooling or freezing of food articles by circulating a cooled stream of air past the articles. In one form of such cooling apparatus, the air stream is directly downwardly through a mass of dry ice particles supported on a perforated bed and the cooled air is then directed to flow around the articles to be treated. While this system is effective, it does require an inordinate amount of time to effect the complete freezing of food articles. For example, the normal food article has to be passed not just once, but generally at least three times, slowly through the stream of cooled air in order to insure that it has been completely frozen. Larger articles would obviously require even more time.
Such prior art systems have the further disadvantage that when the size of the dry ice particles diminishes to a point where they could pass through the support screen, the dry ice particles would fall on the food articles, and if such food articles had been previously cooled and were about ready to be packaged, the dry ice particles would be adhering to the food articles when they were placed in a sealed package. Obviously, the frozen food articles are not maintained at the temperature of dry ice, so that the dry ice would sublimate within the package and quite often cause sufficient swelling of the package to effect its rupture. The cooling efficiency of the process is also reduced by virtue of the fact that the air flow through the mass of dry ice particles is impeded due to the downward packing of the dry ice particles as they decrease in size by sublimation.